The rush to electric power is now in full swing, with automakers scrambling to bring battery models to market. But while much attention is being lavished on improving battery technology, Ricardo engineers are achieving significant efficiency gains through an integrated approach to electric powertrain design.
Electric vehicles have arrived in a big way: big-name launches are a regular occurrence and almost every automaker has one or more plug-in models in its forward product programme. In short, EVs have moved into the mainstream.
For the motor industry, this means a major move away from traditional methods of design and manufacture,
and the new challenge of embracing the potential that an all-electric powertrain offers. New battery electric vehicles (BEVs) provide the opportunity for a fresh approach to design and manufacture – witness the VW Group’s much-promoted MEB modular EV component strategy.
Yet, despite VW’s best efforts, sales of electric powertrain components by established EV manufacturers and
suppliers to third parties have so far failed to take off in anywhere near the numbers needed to accelerate overall EV production. This could indicate the need for a more holistic approach to EV powertrain design, rather than the current practice of specifying a series of individual components. Accordingly, Ricardo has been
thoroughly investigating many different EV powertrain configurations - and the work has led to the design of an innovative ‘three-in-one’, multi-speed Electric Drive Unit (EDU) concept.
“Efficiency is clearly important,” explains Rob Parkinson, Ricardo’s global technical expert for transmission and
driveline systems. “However, cost is paramount if EVs are to be adopted in larger numbers, and it’s fair to say that nobody in the industry is absolutely clear as to which kinds of technology collections are going to be most cost effective overall. That’s fundamental to the work we’re doing now with EDUs.”
Key to developing these new EDU concepts is a Ricardo-developed toolchain consisting of a variety of Ricardo and third-party software packages, all linked together to create a Design of Experiments optimization process capturing electrical, mechanical and thermal energy flows. Interrogating the results from the toolchain gives
clear recommendations as to which powertrain and combination of subsystem technologies is best suited to a
System approach makes
Combining an EV’s electric motor with the transmission and power electronics to make a single unit is an effective way to improve efficiency and reduce cost. Scalable technology of this type could be used by EV manufacturers across a range of EV classes - and an optimum solution that has emerged from Ricardo’s work is a multi-speed EDU incorporating a three-speed dual clutch transmission, electric machine (motor),
final drive and inverter.
Traditional practice has been for the transmission, electric machine and inverter to be developed separately and assembled together, simply because each component has represented a separate, expert discipline. “The
downside of that approach,” explains Parkinson, “is ending up with separate parts that are not necessarily perfectly attuned with one another, whereas designing an integrated system delivers the optimum combination.”
Why opt for a multi-speed rather than single-speed transmission? At first glance a single ratio would seem a less complex and cheaper option. The answer lies in requirements of the electric machine used to power EVs. In a singlespeed solution, the electric machine has to provide both peak torque at low speeds and peak power at higher speeds, through a single gear ratio. In the multi-speed option, explains Parkinson, “a lower ratio
delivers more torque at the axle and a higher ratio reduces the electric machine speed at higher vehicle speeds. An electric machine of a given power can be smaller in terms of torque output as well as dimensions and weight.”
There are also efficiency benefits to be gained by using more than one transmission gear ratio. A traditional
combustion powertrain has brake specific fuel consumption sweet spots or ‘BSFC islands’ in its operating range. “They are quite narrow and constrained within the working range of the engine, ”Parkinson continues, “but with several gear ratios you can use islands in multiple locations throughout the vehicle speed range. With an electric machine, the efficiency island extends much further over the speed range than a combustion engine, but there are still areas where it’s less efficient. Using more than one ratio solves the problem, and we’ve seen efficiency gains of 4 to 5 percent.”
Multi-speed technology does not suit every application, however, and for low-speed, lightweight city cars
with small batteries, says Parkinson, “frankly, you can forget it. A multi-speed won’t give you a benefit, but for a car with a wider working speed range and a bigger battery then, yes, it does.” For the largest vehicles like trucks, it is virtually essential for most applications: “Rather than attempting to make a big truck motor capable of both getting up a hill and cruising at speed on the straight and level, the solution could be to use a smaller motor
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